The invention relates generally to direct current machines and more particularly to direct current machine monitoring systems.
A degraded commutator of a direct current (dc) machine will show excessive sparking as brushes bounce over the rough surface and the conduction period for a particular segment ends prematurely or as a winding remains shorted for too long by a spark region which extends between segments. Ultimately a short circuit will develop, through an extended spark region over the commutator bars, between opposite-polarity brushes. This xe2x80x9cflashoverxe2x80x9d is often severe enough to destroy the machine. Sparking can additionally be caused by factors such as worn down brushes, improper brush positioning, or load or supply problems, for example.
On-line monitoring of commutation quality degradation as a measure of brush and/or commutator degradation or wear, for example, or as a precursor to flashover in dc machines can provide a significant advantage in steel mill, paper mill, and locomotive applications, for example, wherein visual inspection during operation is unsafe or otherwise impractical. As described in Michael P. Treanor and Gerald B. Kliman, xe2x80x9cINCIPIENT FAULT DETECTION IN LOCOMOTIVE DC TRACTION MOTORS,xe2x80x9d Proceedings of the 49th Meeting of the Society for Machinery Failure Prevention Technology, Virginia Beach, Va., April 1995, pp. 221-230, machine condition monitoring for poor commutation can be achieved by frequency-domain analysis of machine current at the frequency of bar passing. The bar-passing frequency can be determined by multiplying the number of commutator bars by the speed (rotation frequency) of the motor. The magnitude of the peak at the bar-passing frequency increased by a factor of at least two when the commutation quality was degraded by incorrectly aligning the brushes. To provide an unambiguous determination of the bar passing frequency, the speed of the motor needs to be obtained with sufficient certainty and precision.
Additionally, once an assessment of commutation quality has been made, it would further be advantageous to assess whether the cause of any degradation resulted from commutator roughness or distortion as compared with non-commutator factors such as brush wear, improper positioning of brushes, or load or supply problems. By identifying the cause, proper maintenance and/or repair can be initiated.
Briefly, in accordance with one embodiment of the present invention, a system for determining a rotation frequency of a direct current machine comprises (a) a sensor for monitoring load current of the machine; and (b) a computer for (i) obtaining a first power spectrum of the load current, (ii) identifying significant peaks of the power spectrum, (iii) identifying one of the significant peaks as indicative of rotation frequency, and (iv) identifying a frequency of the identified peak as the rotation frequency.
In accordance with another embodiment of the present invention, a direct current machine monitoring system comprises (a) a current sensor for monitoring load current of the machine; and (b) a computer for (i) obtaining a power spectrum in a range including a machine trait-passing frequency, (ii) determining a magnitude of a maximum peak in the power spectrum in a range including the trait-passing frequency plus or minus an uncertainty frequency, and (iii) evaluating the magnitude of the maximum peak to assess a condition of the machine.
In accordance with another embodiment of the present invention, a direct current machine monitoring system comprises (a) a current sensor for monitoring load current of the machine; and (b) a computer for (i) obtaining a low frequency power spectrum of the load current, (ii) obtaining at least one magnitude of a component of the power spectrum at a respective predicted frequency, and (iii) evaluating the at least one magnitude of the component to assess a condition of th machine.